1. Field of the Invention
The present invention relates to an optical device including an optical fiber array having one or more optical fibers, and more particularly to an optical device suitable for use in monitoring signal light while the light is propagated through an optical fiber.
2. Description of the Related Art
With the recent development of wavelength multiplex communications using fiber amplifiers, it has become customary to monitor the amounts of light at respective wavelengths with photodiodes (PD), in order to adjust the amounts of light, and then amplify them with amplifiers.
Various methods are known for monitoring amounts of light at respective wavelengths. Since the optical fibers need to be coupled to respective monitoring devices, the monitoring devices alone are of considerable size.
Therefore, there has been a demand for smaller highly packed monitoring devices. For monitoring signal light, it is customary for the monitoring device to extract a portion of the signal light. It is desirable for monitoring devices to be able to monitor signal light without significantly attenuating the signal light.
Heretofore, a technique disclosed in Japanese Laid-Open Patent Publication No. 2001-264594, for example, has been proposed in the art. According to the disclosed technique, an optical fiber is placed in a V-shaped groove defined in a glass substrate, and thereafter, a parallel groove is formed in the glass substrate obliquely to an optical axis of the optical fiber. Then, a light reflecting base (optical member) is inserted into the parallel groove, with the gap filled with an ultraviolet-curable resin (adhesive).
From the signal light that is propagated through the optical fiber, a light component (reflected light) thereof, which is reflected by the light reflecting base, is extracted out of the cladding of the optical fiber. The signal light can be monitored by detecting the reflected light using a photodetector (PD), for example.
If PDs are disposed on the optical fibers, then since most of the optical fibers used are made up from single-core optical fibers, PDs in metal packages are often employed (see, for example, Japanese Laid-Open Patent Publication No. 10-300936, Japanese Laid-Open Patent Publication No. 11-133255, and PCT International Patent Publication No. WO97/06458). This is because single-core optical fibers pose less space limitations, and several PDs in metal packages are already available on the market and have proven satisfactory as to cost and reliability.
However, it is difficult to use PDs in metal packages in combination with multiple-core optical fibers. In particular, if optical fibers are required to be installed at a high density, e.g., at a pitch of 250 μm, then it is necessary to employ a photodiode array (PD array) comprising a plurality of bare photodiodes.
Generally, electric signals from a PD array are output through a package having a plurality of pins.
According to major conventional structures, electrodes are mounted on an upper surface of a V-groove substrate to which an optical fiber array is fixed, or on an upper surface of an optical waveguide that is optically coupled to such a V-groove substrate, and interconnections are connected to a package or the like through the electrodes (see, for example, Japanese Laid-Open Patent Publication No. 7-104146, Japanese Laid-Open Patent Publication No. 2-15203, PCT International Publication No. WO97/06458, and Japanese Laid-Open Patent Publication No. 7-159658).
One common problem is that, since the electrodes are mounted on the upper surface of the V-groove substrate or on the upper surface of the optical waveguide, there are limitations imposed on the interconnections connected (wire bonding) to the PD array, the mounted configuration of the PD array, etc.
In view of the interconnections connected to the package, e.g., wire bonding to the pins of the package, since the wires cannot be too long, the interconnections on the upper surface of the V-groove substrate or on the upper surface of the optical waveguide need to be positioned closely to the pins. This means that the V-groove substrate or the optical waveguide itself needs to be positioned closely to the pins, and the V-groove substrate or the optical waveguide thereby has its width unduly increased. As a result, the cost for such devices is increased.
In addition, it is necessary that the electrodes be formed in relative positional alignment with the V-groove substrate or the optical waveguide. However, a tedious and time-consuming procedure is required to form the electrodes in such a manner.
Furthermore, because the electrodes are mounted on the V-groove substrate after V-shaped grooves are formed therein, or the electrodes are mounted on a completed optical waveguide substrate of the optical waveguide, if the electrodes are defective, then the V-groove substrate or the optical waveguide substrate also becomes defective, resulting in a reduction in yield.
One problem inherent in V-groove substrates, particularly V-groove substrates of glass, is that their surfaces are often ground and do not have a mirror finish. Highly packed interconnections on such rough surfaces are unfavorable from the viewpoints of characteristics and reliability. The surface of a V-groove surface is ground before V-shaped grooves are formed therein because of the need for parallelism between a cutting machine for forming the V-shaped grooves and the machined surface of the substrate. After the surface of the substrate is ground by a surface grinding machine or the like, it is machined to form V-shape grooves therein.
Many conventional arrangements have been disclosed in which, rather than electrodes disposed on the surface of a V-groove substrate or an optical waveguide, the PD array itself is disposed on optical fibers (see, for example, PCT International Publication No. WO97/06458, Japanese Laid-Open Patent Publication No. 63-191111, Japanese Laid-Open Patent Publication No. 2000-249874, and Japanese Laid-Open Patent Publication No. 3-103804). Most of these publications, directed to such conventional arrangements, disclose nothing about interconnections to be connected after the installation of the PD array. Generally, interconnections may be connected to the package via another wiring board disposed between the PD array and the optical fiber array, or by direct wire bonding from the PD array.
Connecting interconnections via another wiring board is troublesome in that the other wiring board must be newly installed (positioned and bonded), after the PD array has been installed, and then the interconnections thereto are connected. Direct wire bonding from the PD array requires considerably long bonding wires, and is not sufficiently reliable. Further, it is highly difficult to perform direct wire bonding because complex interconnections must be directly connected from the PD array, which comprises highly packed photodiodes.
For positioning the PD array, it is the general practice to align the PD array while output currents therefrom are confirmed. According to both of the above processes, since a probe is directly applied to the PD array at this stage, this complicates the positioning process, making it a highly difficult task to perform.
A process involving the connection of interconnections to a PD array is disclosed in Japanese Laid-Open Patent Publication No. 2002-182051, for example. According to the disclosed process, a PD array has a portion installed on a submount and another portion (active layer portion) positioned within an optical path of reflected light, with an adhesive interposed therebetween. That is, a portion of the PD array other than the active layer is installed on the submount, because any obstructions positioned within the optical path leading to the active layer portion of the PD array tend to prevent the assembly from functioning properly as a monitor.
However, in the above structure, when stresses are generated by shrinkage of the adhesive as it is cured, or when dimensional changes thereof occur as it is thermally expanded, the PD array suffers strain stresses, since the PD array itself possesses a moment fulcrum. When such strain stresses are applied to the PD array, the characteristics of the PD array are adversely affected, by an increase in dark current, etc.
The submount supports the PD array thereon and has interconnections formed on its surface. If the submount experiences warpage, however, then the interconnections thereon may become unreliable, and a dark current may be produced by the PD array. Therefore, to avoid such warpage, the submount needs to have a thickness of at least 0.5 mm. Consequently, the distance between the optical fiber array and the PD array cannot be reduced to less than 0.5 mm, resulting in a loss of reflected light and increased crosstalk. In addition, the required distance between the optical fiber array and the PD array poses limitations on efforts to reduce the size of the entire assembly and increase the monitoring capability thereof.